To a large mixing bow add: flour, sugar, salt, baking soda and cocoa powder, and mix till all ingredients are well combined. (If you want to long ferment this batter, do NOT add the baking soda till the next day.)
make a well in the middle of dry ingredients and add the eggs, cream, sourdough starter or discard, and hot water, then whisk till well combined. It will be a thinner batter, and that is correct. (If you are long fermenting this batter cover it now with plastic wrap, and refrigerate for 4-12 hours. Then add your baking soda and stir till combined.)
Preheat your oven to 350 degrees F. And generously butter a 9x13 baking dish.
Pour batter into prepared baking dish.
Bake 9x13 cake pan at 350 for 35-37 minutes... Just till a toothpick inserted in the middle comes out clean.
While cake is baking make the fudgy frosting: Use a double boiler or Make a double boiler: add 1 inch water to a small sauce pan, and place a glass mixing bowl on top (it should fit into the sauce pan an inch or two.)
Add 2 cups chocolate chips and 1 cup heavy cream and stir, or whisk, till well combined and fudgy.
After cake is done, remove from the oven to a cooling rack. Pour warm frosting over hot cake and gently smooth to evenly coat the top.
Let cake cool. And Enjoy!